A light source driving device drives a plurality of light sources. A Power Factor Circuit (PFC) circuit converts a received electrical signal to a DC signal and output to a DC/AC converting circuit. The DC/AC converting circuit converts the DC signal to another AC signal, which is isolated by the transformer circuit. A resonance balancing circuit converts the AC signal output from the transformer circuit to another AC signal to drive the light source module. A PWM dimming controller outputs a control signal to control output of the DC/AC converting circuit according to a received dimming signal, wherein duty cycle of the control signal is fixed. A voltage dividing circuit adjustably divides voltage of the DC signal output from the PFC circuit. A PFC controller feeds the divided signal back to the PFC circuit to control the DC signal output from the PFC circuit.